The present invention generally relates to a gate dielectric for a semiconductor device and method of forming and more particularly, relates to an oxynitride gate dielectric structure that contains nitrided regions with controlled concentrations of nitrogen for use in a semiconductor device and method of forming the structure.
Conventional gate dielectrics used in semiconductor memory devices consist of a thin SiO2 layer. A recent trend in semiconductor processing has been the inclusion of a small concentration of nitrogen in the gate dielectric layer. It has been found that the nitrogen provides the beneficial effects of reducing channel hot electron damage and reducing boron out diffusion from the polysilicon gate into the channel. The inclusion of nitrogen further raises the dielectric constant so that a nitrided film has a lower leakage current than a pure oxide film that has an equivalent capacitance. Even though nitrogen has beneficial effects on gate insulators, too large a nitrogen concentration may be undesirable. Large nitrogen concentrations can cause unacceptable shifts in Vfb, as well as degradation in other properties of the dielectric.
There is little flexibility in the control of the concentration and depth distribution of nitrogen in most methods for introducing nitrogen into SiO2. Chemical methods of introducing nitrogen rely on the reaction between a nitriding agent such as NO or NH3 with the silicon substrate or a previously grown oxide. The resulting films have a large concentration of nitrogen at the Si/SiO2 interface where the chemical reaction takes place. The reaction is self-limiting by the nitrogen since the nitrided layer acts as a diffusion barrier to oxygen and thus prevents further gas species from reaching the Si/SiO2 interface. This provides an additional benefit of using nitrogen in a nitriding process, i.e., the nitrided films are more uniform in thickness than conventional oxides. The uniformity in thickness is demonstrated by the smaller distribution of electrical characteristics measured at different sites across a wafer surface treated with, for example, NO oxidation.
Some limited control of the nitrogen depth distribution has been attempted by others. One of the methods is to control the initial nitridation conditions. For example, it has been shown that the nitrogen content in an oxynitride layer created by exposing silicon to gaseous NO depends on the nitridation temperature. By reacting at a lower temperature, a smaller quantity of nitrogen is introduced, even though the quality of the resulting dielectric may be compromised by the lower reaction temperature. Another method for controlling the nitrogen depth distribution is by reoxidation of oxynitrides. For instance, it has been shown that a pure SiO2 spacer layer can be inserted in between an oxynitride layer and a silicon substrate by exposing the sample to gaseous O2 at elevated temperatures. The oxygen diffuses through the dielectric and reacts with the silicon substrate to form the underlying SiO2 layer without disturbing the oxynitride film. It was also shown that nitrogen can be removed upon reoxidation by N2O, even though it may be more desirable to leave a controlled amount of nitrogen in the oxynitride layer instead of the complete removal such that desirable benefits of nitrogen may be retained. It is therefore desirable to have a method that effectively controls the profile of nitrogen concentration in an oxynitride layer used as a gate dielectric, while simultaneously, after a reoxidation process of the oxynitride layer is carried out, forming a substantially pure SiO2 layer underneath the dielectric.
It is therefore an object of the present invention to provide a method for forming an oxynitride gate dielectric in a semiconductor device that does not have the drawbacks or shortcomings of the conventional methods.
It is another object of the present invention to provide a method for forming an oxynitride gate dielectric in a semiconductor device that is capable of achieving a controlled profile of nitrogen concentration in the oxynitride.
It is a further object of the present invention to provide a method for forming an oxynitride gate dielectric in a semiconductor device that is capable of producing, after a re-oxidation process of oxynitride, a substantially pure SiO2 layer underneath the dielectric.
It is another further object of the present invention to provide a method for forming an oxynitride gate dielectric in a semiconductor device which has a controlled nitrogen profile in the oxynitride layer by first forming the layer by contacting a surface of silicon with at least one gas that contains nitrogen and/or oxygen.
It is still another object of the present invention to provide a method for forming an oxynitride gate dielectric in a semiconductor device that has a controlled profile of nitrogen concentration by first forming the oxynitride layer on a silicon surface by a chemical vapor deposition technique.
It is yet another object of the present invention to provide a method for forming an oxynitride gate dielectric in a semiconductor device by first forming an oxynitride layer and then treating the layer with a gas mixture comprising oxygen and at least one halogenated species for forming a substantially silicon dioxide layer underneath the oxynitride layer.
It is another further object of the present invention to provide a method for forming an oxynitride gate dielectric in a semiconductor device by contacting a surface of silicon with at least one gas that contains nitrogen and/or oxygen selected from the group consisting of NO, N2O, NH3 and O2.
It is yet another further object of the present invention to provide a gate dielectric situated in a semiconductor device that includes a spacer layer of substantially SiO2 overlying a silicon substrate, an oxynitride layer overlying the spacer layer and a SiO2 layer overlying the oxynitride layer.
It is still another further object of the present invention to provide a gate stack situated in a semiconductor memory device that includes a spacer layer of substantially SiO2 overlying a silicon substrate, an oxynitride layer overlying the spacer layer, a silicon dioxide layer overlying the oxynitride layer and a conductive gate overlying the silicon dioxide layer.
In accordance with the present invention, a method for forming an oxynitride gate dielectric that has controlled nitrogen profile in a semiconductor device and gate dielectric formed by such device are provided.
In a preferred embodiment, a method for forming an oxynitride gate dielectric in a semiconductor device can be carried out by the operating steps of first providing a silicon substrate that has a top surface, and contacting the top surface of the silicon substrate with at least one gas that contains nitrogen and/or oxygen at a temperature of not less than 500xc2x0 C. forming an oxynitride layer overlying the silicon substrate, and contacting the silicon substrate and the oxynitride layer with a gas mixture including oxygen and at least one halogenated species such that a layer of substantially silicon dioxide is formed between the oxynitride layer and the silicon substrate.
The at least one gas that contains nitrogen and/or oxygen can be selected from the group consisting of NO, N2O, NH3 and O2. The at least one gas may be flown into a reaction chamber to react at a pressure between about 1 mTorr and about 20 atm. The at least one gas may be flown into a reaction chamber at a sufficiently high flow rate and chamber temperature such that a nitrogen-rich oxynitride having a nitrogen concentration in the range between about 0.1 and about 50 atomic percent can be formed. The at least one gas may be flown into a reaction chamber to react with a silicon surface that is kept at a temperature between about 500xc2x0 C. and about 1200xc2x0 C.
The method for forming an oxynitride gate dielectric may further include the step of contacting the top surface of a silicon substrate by at least one nitrogen-containing gas and at least one oxygen-containing gas. The method may further include the step of contacting a top surface of a silicon substrate with at least one gas selected from NO, N2O and NH3 and at least one gas selected from O2 and N2O. The oxynitride layer formed may have a thickness between about 1 xc3x85 and about 40 xc3x85. The at least one halogenated species may be selected from the group consisting of HCl, CH2C2, C2H3C3, C2H2C2. CH3Cl and CHCl3. The layer of substantially silicon dioxide may be formed of 90% pure SiO2.
In another preferred embodiment, a method forming an oxynitride gate dielectric may be carried out by first providing a silicon substrate which has a top surface, depositing a layer of oxynitride on the top surface of the silicon substrate by a chemical vapor deposition technique, and then forming a substantially silicon dioxide layer between the oxynitride layer and the silicon substrate by contacting the oxynitride layer with a gas mixture including oxygen and at least one halogenated species.
The chemical vapor deposition (CVD) technique utilized may be plasma CVD, remote plasma CVD, rapid thermal CVD and low pressure CVD. The oxynitride layer deposited may have a thickness between about 1 xc3x85 and about 40 xc3x85. The at least one halogenated species may be selected from the group consisting of HCl, CH2Cl2, C2H3Cl3, C2H2C2. CH3Cl and CHCl3. The layer of substantially silicon dioxide may be formed of 90% pure SiO2.
The present invention is further directed to a gate dielectric situated in a semiconductor device that includes a silicon substrate, a spacer layer overlying the silicon substrate, the spacer layer may be formed of substantially SiO2, an oxynitride layer overlying the spacer layer, and SiO2 overlying the oxynitride layer.
In the gate dielectric formed, the oxynitride layer may be a nitrogen-rich oxynitride which has a nitrogen concentration in the range of between about 0.1 and about 50 atomic percent. The thickness for the spacer layer, the oxynitride layer and the SiO2 may be between about 1 xc3x85 and about 40 xc3x85. The spacer layer formed may contain 90% pure SiO2 
The present invention is further directed to a gate stack situated in a semiconductor memory device that includes a silicon substrate, a spacer layer overlying the silicon substrate wherein the spacer layer may be formed of substantially pure SiO2, an oxynitride layer overlying the spacer layer, a silicon dioxide layer overlying the oxynitride layer and a conductive gate overlying the silicon dioxide layer.
In the gate stack formed in a semiconductor memory device, the conductive gate may be a polysilicon gate. The oxynitride layer may be a nitrogen-rich oxynitride which has a nitrogen concentration in the range between about 0.1 and about 50 atomic percent. Each of the spacer layer, the oxynitride layer and the SiO2 layer may have a thickness between about 1 xc3x85 and about 40 xc3x85. The spacer layer may be formed of a material that is substantially SiO2.